In radio, infrared, cable, fiber optics and practically all communication transmission systems, power and spectral efficiency combined with robust bit error rate (BER) performance in a noisy and/or strong interference environment is a most desirable system requirement. Robust BER performance is frequently expressed in terms of the BER as a function of Energy per Bit (Eb) divided by Noise Density (No), that is, by the BER=f(Eb/No) expression. Cost, reduced size, compatibility and interoperability with other conventional or standardized systems is highly desired. Several recently-developed draft standards have adopted modulation techniques such as conventional binary phase shift keying (BPSK), quadrature phase shift keying (QPSK), and pi/4-QPSK techniques including differential encoding variations of the same. For spectrally-efficient (i.e., band limited) signaling, these conventional methods exhibit a large envelope fluctuation of the modulated signal, and thus a large increase in peak radiation relative to the average radiated power. Within the present state of the art, for numerous transmitter applications, it is not practical to introduce bandpass filtering after the radio frequency (RF) final amplifier stage. Here we are using the term “radio frequency” in its broadest sense, implying that we are dealing with a modulated signal. The RF could be, for example, as high as the frequency (inverse of the wavelength) of infrared or fiber optic transmitters, it could be in the GHz range, e.g., between 1 GHz and 100 GHz, or it could be in the MHz range or just in the kHz range.
In conventional BPSK and differentially-encoded phase-shift keying systems such as DBPSK and DQPSK, large envelope fluctuations require linearized or highly linear transmitters including up converters and RF power amplifiers and may require expensive linear receivers including linear automatic gain control (AGC) circuits. A transmitter nonlinear amplifier (NLA) reduces the time domain envelope fluctuation of the band limited signal and this reduction of the envelope fluctuation, being a signal distortion, is the cause of spectral restoration or spectral regrowth and the cause of unacceptable high levels of out-of-band spectral energy transmission, also known as out-of-band interference. Additionally, for conventional BPSK, QPSK, and also QAM (Quadrature Amplitude Modulation) signals, in phase channel (I) to quadrature channel (Q) crosstalk is generated which degrades the BER=f(Eb/No) performance of the modulated radio transmitter.
Experimental work, computer simulation, and theory documented in many recent publications indicates that for band limited and standardized BPSK, QPSK, pi/4-QPSK, and QAM system specifications, a highly linear amplifier is required to avoid the pitfalls of spectral restoration and of BER degradation. Linearized or linear amplifiers are less power efficient (during the power “on” state, power efficiency is defined as the transmit RF power divided by DC power), considerably more expensive and/or have less transmit RF power capability, are larger in size, and are not as readily available as NLA amplifiers. As an illustrative example of technology achievements on two recently-released radio frequency integrated circuit (RFIC) amplifiers, we measured a maximum possible output power of 18 dBm in a linear mode of operation and as much as 24 dBm in a nonlinear or saturated mode of operation practically with the same DC current and DC power requirement at 2.4 GHz (Minicircuits amplifier MRFIC 2403). The RF power to DC drive power ratio, which is a practical measure of power efficiency of an RF output stage, was doubled in the saturated mode. The reduced linearly amplified output power of 18 dBm is required to meet the stringent spectral efficiency requirements of the IEEE 802.11 direct sequence spread spectrum draft standard for conventional DBPSK and DQPSK operation, as depicted in FIG. 1. From FIG. 1 note that the linearly amplified filtered BPSK signal could meet the spectral specifications of this 11M chip/second system. The nonlinearly amplified, filtered BPSK of the prior art does not meet the specifications. The power efficiency (RF power to DC power ratio) of these systems in the “on” mode with linear low-cost commercial amplifiers driven by a 3 V battery was found to be as low as 10%.
In an extremely critical power-efficient requirement such as all wireless and cellular telephones, computers, and other devices, it is very wasteful to operate at such low power efficiency, which leads to frequent replacement of the battery. The so-called “talk time” is not efficient with these very recently standardized IEEE 802.11 modulated conventional BPSK and DQPSK systems. As a specific example, the out-of-band power spectral density is specified by the IEEE 802.11 U.S. and international standard to be attenuated at least 30 dB at 11 MHz away from the carrier frequency for an 11M-chip/sec system as illustrated in FIG. 1, by the shaded area of specification limits. In simple modulated signal terms the 11 M-chip/sec rate is similar to an 11M-chip/sec DBPSK modulator. To satisfy this -30 dB out-of-band spectral density requirement, an “output back off” (OBO) of the RF amplifiers of 4 dB to 6 dB is required. See K. Feher, “Wireless Digital Communications: Modulation and Spread Spectrum Techniques,” book, Prentice Hall, 1995, and H. Mehdi, K. Feher, “FBPSK, Power and Spectrally Efficient Nonlinearly Amplified (NLA) Compatible BPSK Modems for Wireless LAN” submitted to RF Expo 95 San Diego proceedings to be published, Mar. 1995 and H. Mehdi, K. Feher, “FQPSK, Power and Spectral Efficient Family of Modulations For Wireless Communication System.” Proceeding IEEE-VTC-94, Jun. 1994, FIG. 2a depicts DBPSK modulation utilizing an amplifier (MRFIC 2403 available from Motorola) with an output power of approximately 18.5 dBm, which is linearly amplified with an output-back off (OBO) of 5 dB.
FIG. 2b depicts a pre-modulation filtered conventional DBPSK signal operated at saturation at approximately 24 dBm, in which spectral restoration is evident. In FIG. 2c, the FBPSK modulated signal power is 24 dBm at full saturation, with 0 dB OBO. The term OBO is for the output power reduction required from the maximal or saturated output power of the amplifier. In this case saturated output power corresponds to 24 dBm while the 6 dB OBO reduces the output power from 24 dBm to only 18 dBm. The DC power consumption of the evaluated RF devices did not change significantly between the saturated full-output RF power and the 6 dB OBO reduced-output power. Thus if we could have a modulated system which could operate at full saturation of 24 dBm and meet the standardization requirements and specifications we could achieve approximately a 6 dB (400%) increased output power. Thus a modulation technique which could attain 24 dBm from existing RF devices and meet the standardized system specifications as well as the desired compatibility with conventional standardized BPSK or QPSK systems would be very attractive. To put things even more into perspective for this illustrative application, at 2.4 GHz FCC Part 15 permits transmission of 1 watt=+30 dBm transmit power. The IEEE 802.11 Standardization Committee specifies the same +30 dBm maximal output power. In a strongly interference polluted environment of the unregulated FCC-15 band it is very desirable to transmit the strongest possible and permitted RF signal in order to achieve good performance and the best possible coverage. With conventional BPSK modulators (such as specified by IEEE 802.11) utilizing linearly operated amplifier devices (such as a MRFIC 2403 amplifier available from Motorola) the practical achievable limit of output power is around +18 dBm to +20 dBm, which is about 10 dB less than permitted by the FCC and the IEEE standard. Thus, due to technology limitations, conventional modems must operate at an approximately 10 dB lower linearly amplified output power than permitted by FCC and allowed by the IEEE specification, for best performance.